Glycols such as mono-ethylene glycol (MEG), mono-propylene glycol (MPG) and 1,2-butanediol (1,2-BDO) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET or polyester. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focussed on producing chemicals, including glycols, from non-petrochemical renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols can be seen as an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to glycols revolve around a two-step process of retro-aldol fragmentation and hydrogenation, as described in Angew, Chem. Int. Ed. 2008, 47, 8510-8513. Sponge metal catalysts such as Raney nickel are often used as the hydrogenation catalyst in such processes. Some amounts of leaching may occur with these catalysts and such leaching can lead to the presence of metal in the product, or could lead to catalyst deactivation.
WO2015028398 describes a continuous process for the conversion of a saccharide-containing feedstock into glycols. In this process the saccharide-containing feedstock is contacted in a single reactor with two catalysts, hydrogen and a solvent. For example, a solution of glucose in water is contacted with a W/Ni/Pt on zirconia catalyst and a Ru on silica catalyst in the presence of hydrogen. The former catalyses the initial conversion of the saccharide-containing feedstock into retro-aldol fragments (e.g. glycolaldehyde and hydroxyacetone), and the latter coverts such fragments to MEG, MPG and 1,2-BDO. The temperature of the reaction is typically 195° C. and the absolute pressure is typically around 7.5 MPa, however, often the reaction temperature may need to be in the region of around 220° C. to 240° C. to drive the initial conversion of the saccharide-containing feedstock into retro-aldol fragments.
U.S. Pat. No. 4,503,274 discloses the inorganic oxide catalyst supports alpha-alumina, theta-alumina, titanated alumina and titania, and describes hydrogenating a carbohydrate in aqueous solution to its polyols at a preferred temperature range of from about 105° C. to 130° C. The catalyst used in such reaction is a catalyst consisting essentially of zerovalent ruthenium dispersed on theta-alumina support, where the zerovalent ruthenium is produced by the reduction of the impregnated ruthenium on the on theta-alumina support with hydrogen at a temperature from about 100° C. to about 300° C.
As described by its inventors, the catalyst of U.S. Pat. No. 4,503,274 appears to have better hydrothermal stability than its prior art, in particular when an aqueous solution of carbohydrates is treated with hydrogen at hydrogenation conditions of a preferred reaction temperature of 105 to 130° C. However, the present inventors have observed that in the hotter aqueous conditions which they use (which may be up to 250° C.) for the production of glycols from saccharide-containing feedstock, such catalyst compositions are not hydrothermally stable, probably due to the inorganic oxide catalyst supports. For example, such catalyst supports may undergo phase changes or growth of crystallites, or may begin to dissolve. This can detrimentally affect catalytic performance, leading to lower glycol yields, and the need to change the catalyst more frequently. This can also lead to system instability such that reaction conditions may need to be changed to maintain catalyst performance. Additionally, dissolution of catalyst components can lead to the presence of impurities in the glycol process.
Typically, a first step of the known processes for the preparation of supported catalysts is the impregnation of catalyst supports with the ionic form of catalytically active metal(s) of choice. A common practice after impregnation is to dry the support to remove the solvent used in the impregnation step. After this, commonly a heat treatment step, in either an oxygen- or a hydrogen-containing atmosphere is carried out. Following the treatment by any one of these gasses, further steps are commonly required before the catalyst can be ready for its intended use. If oxygen-treated, typically a further step of reduction is required to convert the impregnated metal ion to its catalytically active metallic state. If hydrogen-treated, the reduced metal particles usually become pyrophoric, so they then require a further step of passivation before they can be exposed to an oxygen containing atmosphere, such as air. During passivation, a protective surface coating, usually comprising the metal's oxide, is formed by subjecting the metal particles to a low concentration of oxygen in a temperature- and oxygen concentration controlled manner known in the art. Following the passivation step, a second reduction step needs to be carried out before the catalyst can be ready for its intended use.
Both these gas treatments affect the resultant catalyst's activity profile and physical properties differently, with each having its own disadvantages. Some of the disadvantages of the oxygen-treatment are, for example, the vulnerability of supports comprising carbon to burn during the oxygen-treatment, or the promotion of particle size growth of the impregnated metal by sintering. In the case of hydrogen-treatment, the main disadvantage is the formation of the pyrophoric metal particles, which without the passivation step would begin to heat up in the presence of an oxygen containing atmosphere, such as air, and lead to metal particle growth.
A further disadvantage of the known processes for the preparation of supported catalysts is the lack of hydrothermal stability of the catalyst compositions that such support impregnation processes produce. Such hydrothermal instability can be seen, in particular, when such a catalyst composition is used under the conditions of the continuous conversion of a saccharide-containing feedstock into glycols described herein.
Overall, prior art support impregnation processes are complicated by their complexity, as they comprise numerous and alternative process steps, and their resultant catalyst products are not hydrothermally stable and active under some conditions. Therefore, a process which reduces the complexity and the number of steps of the impregnation process, and one that produces a catalyst that is hydrothermally stable and active across a wider range of conditions would be desirable.
Therefore the present inventors have sought to obtain a hydrothermally stable catalyst composition suitable for one or more steps of the continuous conversion of a saccharide-containing feedstock into glycols, and have discovered that the catalyst composition making process described herein not only makes a hydrothermally stable active catalyst for the conversion of a saccharide-containing feedstock into glycols, but also one which is quicker and easier to prepare compared to the processes of the prior art.